The present invention relates to serial data link system analysis, and more particularly to a serial data link measurement and simulation system analysis interface.
Currently high speed signaling for a number of designs and standards for serial data links has become severely channel, or interconnect, limited. The result is that an eye diagram, which represents a digital signal transmitted over the serial data link, is often closed at the receiver. Equalization methods are used both for the transmitter as well as for the receiver to open the eye diagram and operate the serial data link to assure accurate transfer of digital information from a transmitter to the receiver.
There are several challenges when measuring or debugging the serial data link when operating on a severely limited channel: the channel and the transmitter interact in a complex way; the eye diagram at the receiver is nearly or completely closed; the receiver doesn't discriminate the digital signal directly but only after equalization; etc. FIG. 1 represents a serial data link having a transmitter at one end with transmitter equalization, a transmitter connector, a cable, a receiver connector and a receiver with receiver equalization. As indicated by the associated signal diagrams, the output from the transmitter has a very clear eye diagram, while the signal at the receiver appears to have no discernible eye diagram prior to equalization. The equalization attempts to recreate the eye diagram as transmitted from the transmitter, but the result is not ideal.
What is tested in such a serial data link is the transmitter plus channel combination which may include a transmitter (Tx) printed circuit board (PCB), a cable with its connectors, and a receiver (Rx) printed circuit board. The Rx is usually represented by an equalizer emulator in a test instrument, such as an oscilloscope. However, the connection of the test instrument to the transmitter, or to any point within the serial data link, presents its own distortions which need to be accounted for in order to produce reliable, usable measurements.
Many test and measurement manufacturers, such as Tektronix, Inc. (Tektronix), Agilent Technologies (Agilent) and Teledyne LeCroy (LeCroy), have had serial data link measurement systems for several years. However, none has represented the system in a way that totally and properly ties together all aspects of the serial data link system. These systems generally are based upon S-parameter block models, where S-parameters are ratios that represent how much of a signal introduced at an input port of a block is either reflected back to the input port, is passed through to an output port, appears as cross-talk (coupling between lines having different signals) on an adjacent channel within the block, the adjacent channel having its own input and output ports, or is cross-coupled (mutual coupling between two lines of A and B components representing the same signal) to an adjacent signal line.
For example, LeCroy has a serial data link package named Eye Doctor which uses a full S-parameter block schematic diagram to model the serial data link system. This system does allow full S-parameter modeling, but it does not tie it all together with the overall serial data link model, making it somewhat complex and difficult for a user.
LeCroy also has a subsequent serial data link package, Eye Doctor II, which uses a digital signal processing (DSP) signal flow model rather than a left-to-right virtual layout of the serial data link channel. This model does not completely represent cross-talk, cross-coupling or full cascading of S-parameters with the menus used. Detail of the processing flow is still somewhat obscure and confusing.
The current Tektronix serial data link package incorporates de-embedding of a fixture block, representing the disturbances caused by the connection of the test instrument, and the embed of a channel block. The Tektronix package incorporates a menu which shows the serial data model flow, but presents the fixture block as hanging in space with a wire, suggesting the connection test point but not the actual connection. The associated menu view is not optimal for the user in showing how things are connected and how the signal processing flow is implemented. However, this model does not correctly represent cross-talk, cross-coupling or full cascading of S-parameters from block to block.
Agilent has a serial data package that has various pieces of the serial data link model features scattered through various test instrument menus. It is not tied together into one complete coherent system menu. The portion that does S-parameter modeling correctly represents the measured part of the system S-parameter blocks compared to the simulated portion of the blocks. However, the Agilent package combines the de-embed and embed blocks into one block each, which may be correct but is difficult to look at and understand since each block must be defined twice, once for the measurement circuit and once for the simulation circuit.
What is desired is a serial data link measurement and simulation system that is easier to use and properly represents cascaded S-parameters, AMI models, and cross-talk measurement and simulation. AMI stands for Algorithmic Modeling Interface, which is a recognized standard for receiver circuit function description.